1. Field of the Invention
This invention relates to block copolymers having at least 2 blocks and utilizing a mixture of cis and trans-isomers of 1,3-pentadiene. More specifically these block copolymers having 2 or more blocks utilize a mixture of cis and trans-isomers of 1,3 pentadiene and a cis-isomer of 1,3-pentadiene polymerizing amounts of a polymerization promoter to increase the amount of total pentadiene polymerized.
2. Description of the Prior Art
Block copolymers of vinyl aromatic compounds and conjugated dienes, principally styrene and butadiene and isoprene are not new in the art. U.S. Pat. No. 3,265,765 to Holden et al. teach block copolymers having the general configuration A-B-A where each A is a monovinyl aromatic hydrocarbon polymer block and B is an elastomeric conjugated diene polymer block. These block copolymers are unvulcanized. U.S. Pat. No. 3,465,065 to Moss et al. teaches the preparation of block copolymers utilizing particular polymerization solvents. These block copolymers have the general configuration A-B-C-B-A where A is a ploymer block of a monovinyl substituted aromatic compound, B is a polymer block of a reacted conjugated diene and C is a monomeric residue of a coupling agent joining the two B blocks. This reference states that one of the difficulties encountered in the production of block copolymers is the presence in normal commercial feeds of undesirable homologues and analogues of desired monomers. Specifically, this reference states isoprene feeds are frequently contaminated with 1,3-pentadiene or piperylene, which causes difficulty in the formation of the third polymer block of the subject block copolymers. According to the reference, the polymerization of that particular polymer chain virtually ceases due to the low rate of addition of piperylene.
In utilizing a mixture of cis and trans-isomers as the conjugated diene to form block copolymers, it has been found that the cis-isomer of 1,3-pentadiene polymerizes negligibly if at all when forming an elastomer center block of an A-B-A configuration block copolymer. This cis-isomer of 1,3-pentadiene residue decreases the total yield of 1,3-pentadiene polymerized. Even more significant, the cis-isomer of 1,3-pentadiene residue inhibits the formation of the third polymerized vinyl-substituted aromatic hydrocarbon block unless living chains of intermediate block copolymers having the configuration A-B-Li are joined with other chains having a similar configuration through the use of a coupling agent.
While a coupling agent solves the problem of polymerization inhibition due to the residue of cis-isomer of 1,3-pentadiene, it does not solve the non-use of the cis-isomer of 1,3-pentadiene in a block copolymer.